Biaxially oriented polyester film and method for production thereof

ABSTRACT

A biaxially oriented polyester film for a high-density magnetic recording medium, particularly a high-density magnetic recoding medium of linear recording system, which has high strength in longitudinal and transverse directions, excellent dimensional stability in a crosswise direction and is flat and excellent in output characteristics and a process for producing the same. The biaxially oriented film is made from polyethylene-2,6-naphthalate and has a Young&#39;s modulus in a longitudinal direction of 8 GPa or more, a Young&#39;s modulus in a transverse direction of 6 GPa or more, a temperature expansion coefficient in the transverse direction (αt) of −5×10 −6 /° C. to +12×10 −6 /° C., a humidity expansion coefficient in the transverse direction (αh) of 5×10 −6 /%RH to 12×10 −6 /%RH and a thermal shrinkage factor in the transverse direction at 105° C. of −0.5 to +1.5%

FIELD OF THE INVENTION

[0001] The present invention relates to a biaxially oriented polyesterfilm and a production process therefor. More specifically, it relates toa biaxially oriented polyester film which has excellent dimensionalstability while retaining high Young's moduli and is therefore useful asa base film for high-density magnetic recording media, particularly LTOand S-DLT magnetic tapes of linear recording system, and a productionprocess therefor.

DESCRIPTION OF THE PRIOR ART

[0002] A polyester film is used in a wide variety of fields such asmagnetic recording media and electrical insulation as it has excellentthermal and mechanical properties. As the capacity and density of amagnetic recording medium, particularly a data storage tape have beenincreasing in recent years, requirements for a base film for use in themedium have been becoming higher and higher.

[0003] In order to ensure a large capacity for a tape, it is conceivableto reduce the thickness, extend the length, flatten the magnetic side orincrease the linear recording density or the number of tracks of thetape. A base film having higher flatness, higher strength and excellentdimensional stability in a crosswise direction is desired.

[0004] Heretofore, a polyethylene terephthalate film has been widelyused as a base film for magnetic tapes but apolyethylene-2,6-naphthalene dicarboxylate film having high strength andhigh dimensional stability has recently been used very often. However,when the strength in the longitudinal direction of the film is to beincreased, the strength in the transverse direction lowers, resulting indeteriorated dimensional stability in the crosswise direction. Also,when the strength in the transverse direction is to be increased toimprove dimensional stability in the transverse direction, the strengthin the longitudinal direction lowers. Thus, a biaxially orientedpolyethylene-2,6-naphthalene dicarboxylate film which has excellentdimensional stability while retaining high Young's moduli in bothlongitudinal and transverse directions is yet to be provided.

SUMMARY OF THE INVENTION

[0005] It is an object of the present invention to provide a biaxiallyoriented polyester film which solves the above problem, has highstrength in both longitudinal and transverse directions and excellentdimensional stability in the crosswise direction and comprisesethylene-2,6-naphthalene dicarboxylate as the main recurring unit.

[0006] It is another object of the present invention to provide a flatbiaxially oriented polyester film which is useful as a base film forhigh-density magnetic recording media having excellent outputcharacteristics, particularly high-density magnetic recording media oflinear recording system.

[0007] It is still another object of the present invention to provide anindustrially advantageous process for producing a biaxially orientedpolyester film having the above excellent properties of the presentinvention.

[0008] Other objects and advantages of the present invention will becomeapparent from the following description.

[0009] According to the present invention, firstly, the above objectsand advantages of the present invention are attained by a process forproducing a biaxially oriented polyester film, comprising the steps of:

[0010] (1) stretching an unstretched film of a polyester which comprisesethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol %of the total of all the recurring units to 4.5 to 7.0 times in a machinedirection at a temperature of 100 to 190° C. to form a uniaxiallyoriented film; and

[0011] (2) stretching this uniaxially oriented film to 4.0 to 7.0 timesin a transverse direction at a temperature of 110 to 170° C. whileraising the temperature in the traveling direction of the film and thenstretching the film to 1.05 to 1.5 times at a lower draw rate than thefirst draw rate at a temperature from the final temperature of the firsttransverse orientation to 240° C. while raising the temperature in thetraveling direction of the film to form a biaxially oriented film having(i) a Young's modulus in the longitudinal direction of 8 GPa or more,(ii) a Young's modulus in the transverse direction of 6 GPa or more,(iii) a temperature expansion coefficient in the transverse direction(αt) of −5×10⁻⁶/° C. to +12×10⁻⁶/° C., (iv) a humidity expansioncoefficient in the transverse direction (αh) of +5×10⁻⁶/%RH to+12×10⁻⁶/%RH, and (v) a thermal shrinkage factor at 105° C. in thetransverse direction of −0.5 to +1.5%.

[0012] According to the present invention, secondly, the above objectsand advantages of the present invention are attained by a biaxiallyoriented polyester film which has (i) a Young's modulus in alongitudinal direction of 8 to 12 GPa, (ii) a Young's modulus in atransverse direction of 6.5 to 9 GPa, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +6×10⁻⁶/%RH to +12×10⁻⁶/%RH and (v) a thermalshrinkage factor at 105° C. in the transverse direction of 0 to +1.5%and which comprises (vi) ethylene-2,6-naphthalene dicarboxylate in anamount of at least 95 mol % of the total of all the recurring units.

BRIEF DESCRIPTION OF THE DRAWING

[0013]FIG. 1 is a schematic diagram of a device for measuring adimensional change in a crosswise direction under load in a longitudinaldirection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] The present invention will be described in detail hereinbelow. Adescription is first given of the production process. As describedabove, the process of the present invention comprises the steps ofstretching a film in a machine (longitudinal) direction and thenstretching the film in a transverse direction in two stages, the drawrate of the second-stage transverse orientation being lower than thedraw rate of the first-stage transverse orientation.

[0015] The unstretched film to be uniaxially stretched in the step (1)is made from a polyester which comprises ethylene-2,6-naphtahlenedicarboxylate in an amount of at least 95 mol % of the total of all therecurring units. The polyester is particularly preferably a homopolymerof ethylene-2,6-naphthalene dicarboxylate.

[0016] The polyester can be produced by a method known per se. Forexample, it can be produced by carrying out an ester exchange reactionbetween a lower alkyl ester of 2,6-naphthalenedicarboxylic acid andethylene glycol and polycondensing the reaction product. The esterexchange reaction catalyst used for the ester exchange reaction ispreferably a manganese compound, and the manganese compound ispreferably an oxide, chloride, carbonate or carboxylate, particularlypreferably manganese acetate. When the ester exchange reaction issubstantially completed, a phosphorus compound is preferably added todeactivate the ester exchange catalyst. The phosphorus compound ispreferably trimethyl phosphate, triethyl phosphate, tri-n-butylphosphate or orthophosphoric acid, particularly preferably trimethylphosphate. The polycondensation catalyst is preferably an antimonycompound, particularly preferably antimony trioxide.

[0017] The intrinsic viscosity of the thus obtained polyester ispreferably 0.40 (dl/g) or more, more preferably 0.40 to 0.90. When theintrinsic viscosity is less than 0.4, the film often breaks in thestretching step. When the intrinsic viscosity is more than 0.9, thepolymerization productivity of the polyester tends to lowerdisadvantageously. After melt polymerization, the polyester may bechipped and solid-phase polymerized under vacuum heating or in a streamof an inert gas such as nitrogen.

[0018] Various additives may be added to the above polyester used in thepresent invention in limits not prejudicial to the object of the presentinvention. Particularly addition of inert fine particles is desired toadjust the surface roughness of the obtained biaxially orientedpolyester film to a suitable range. Addition of the inert fine particleswill be described hereinafter.

[0019] In the step (1), the unstretched film of the above polyester isstretched to 4.5 to 7.0 times in the machine direction at a temperatureof 100 to 190° C. to form a uniaxially oriented film. When the drawratio stretching in machine direction is lower than 4.5 times, theYoung's modulus in the longitudinal direction of the finally obtainedbiaxially oriented film tends to fall below 8 GPa and when the drawratio stretching in machine direction is higher than 7.0 times, the filmis easily broken by stretching in the transverse direction in thesubsequent step (2), thereby making it difficult to adjust the Young'smodulus in the transverse direction of the finally obtained biaxiallyoriented film to 6 GPa or more.

[0020] The temperature for stretching in machine direction of the step(1) is preferably 120 to 170° C. and the draw ratio is preferably 5.0 to6.5 times.

[0021] The uniaxially oriented film obtained in the step (1) has arefractive index in the longitudinal direction (NMD) of 1.77 or more, arefractive index in the transverse direction (NTD) of 1.55 to 1.62,preferably 1.57 to 1.60, and a refractive index in the thicknessdirection (NTD) of preferably 1.50 to 1.56, particularly preferably 1.52to 1.56. When the refractive indices of the uniaxially oriented film areoutside the above ranges, the film is often broken by stretching in thetransverse direction in the subsequent step, or a biaxially orientedfilm having the targeted Young's moduli is hardly obtained.

[0022] In the step (2), the uniaxially oriented film is first stretchedto 3.0 to 6.0 times in the transverse direction at 110 to 170° C. whilethe temperature is raised in the traveling direction of the film,namely, the temperature of the film is raised in the traveling directionof the film (first transverse stretching). When the above draw ratio inthe transverse direction is lower than 3 times, a biaxially orientedfilm having the targeted Young's modulus in the transverse direction ishardly obtained and when the draw ratio is higher than 6 times, the filmis often broken, resulting in greatly deteriorated film formingproperties.

[0023] The temperature of the first transverse orientation in the step(2) is preferably 130 to 160° C. and the draw ratio is preferably 4.0 to5.0 times. As for the elevation of the temperature in the travelingdirection of the film for the first transverse stretching, the gradientof a temperature rise is preferably in the range of 15 to 55° C., morepreferably 20 to 50° C. during the first transverse orientation.

[0024] In the step (2), the first transverse stretching is followed byfurther stretching the film to 1.05 to 1.5 times in the transversedirection at a lower draw rate than the draw rate of the firsttransverse stretching at a temperature from the same temperature as thefinal temperature of the first transverse stretching to 240° C. whileraising the temperature in the traveling direction of the film (secondtransverse stretching). When the draw ratio of the second transversestretching is lower than 1.05 times, the draw ratio of the previoustransverse stretching must be made excessively high to obtain thetargeted Young's modulus in the transverse direction of the finallyobtained biaxially oriented film. In this case, the film is readilybroken by the previous transverse stretching. When the draw ratio of thesecond transverse stretching is higher than 1.5 times, the thermalshrinkage factor in the transverse direction of the obtained biaxiallyoriented film becomes too large disadvantageously.

[0025] The temperature of the second transverse stretching is preferablyfrom the same temperature as the final temperature of the firsttransverse stretching to 220° C. and the draw ratio is preferably 1.05to 1.2 times.

[0026] As for the elevation of the temperature in the travelingdirection of the film for the second transverse stretching, the gradientof a temperature rise is preferably in the range of 20 to 90° C., morepreferably 25 to 85° C. during the second transverse orientation.

[0027] The draw rate of the second transverse stretching is lower thanthe draw rate of the first transverse stretching which is 10 to300%/sec, for example, 0.1 to 30%/sec. The ratio of the draw rate of thefirst transverse stretching to the draw rate of the second transversestretching is preferably 0.005 to 0.5, more preferably 0.01 to 0.3, muchmore preferably 0.01 to 0.1.

[0028] In the process of the present invention, after the secondtransverse stretching, it is preferred to further carry out the step ofshrinking or stretching the film to 0.9 to 1.05 times in the transversedirection at the same temperature as the final temperature of the secondtransverse orientation or at a temperature of 170 to 230° C. whilereducing the temperature from the final temperature in the travelingdirection of the film. When the temperature of this step, that is, heatsetting or crystallization step is lower than 170° C., the thermalshrinkage factor in the transverse direction of the film at 105° C.becomes too large and may exceed 1.5%. When the temperature is higherthan 230° C., the temperature and humidity expansion coefficients becomelarge and the dimensional stability in the crosswise direction isdeteriorated by temperature and humidity variations. As for adimensional change in the crosswise direction caused by final heatsetting, when the toe-out (elongation) is more than 10%, the thermalshrinkage factor becomes large, thereby deteriorating dimensionalstability in the crosswise direction and when the toe-in (shrinkage) ismore than 5%, the Young's modulus in the transverse direction of thefilm is suddenly lowered by this heat setting, thereby making itdifficult to obtain the required Young's modulus in the transversedirection. The total area draw ratio is preferably 20 to 50 times, morepreferably 25 to 45 times, particularly preferably 30 to 40 times.

[0029] As described above, by the process of the present invention, abiaxially stretched (oriented) film having (i) a Young's modulus in thelongitudinal direction of 8 GPa or more, (ii) a Young's modulus in thetransverse direction of 6 GPa or more, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +5×10⁻⁶/%RH to +12×10⁻⁶/%RH, and (v) a thermalshrinkage factor at 105° C. in the transverse direction of −0.5 to +1.5%is formed.

[0030] The above biaxially oriented film is characterized in that itsYoung's modulus in the longitudinal direction, Young's modulus in thetransverse direction, temperature expansion coefficient in thetransverse direction (αt), humidity expansion coefficient in thetransverse direction (αh) and thermal shrinkage factor at 105° C. in thetransverse direction are within the respective specific ranges.

[0031] This biaxially oriented film has a Young's modulus in thelongitudinal direction of the film of 8 GPa or more and a Young'smodulus in the transverse direction of the film of 6 GPa or more. Whenthe Young's modulus in the longitudinal direction is lower than 8 GPaand strong stress is applied to a magnetic tape, the tape elongates inthe longitudinal direction and deforms disadvantageously. When the filmis used in a magnetic recording medium of linear recording system havinga high track density, the medium shrinks in the crosswise direction byits elongation in the lengthwise direction, thereby causing trackdislocation. The Young's modulus in the longitudinal direction ispreferably 8.5 GPa or more, more preferably 9 GPa or more.

[0032] When the Young's modulus in the transverse direction of the filmis lower than 6 GPa, the temperature and humidity expansion coefficientsin the transverse direction become large, whereby when it is used in amagnetic recording medium of linear recording system having a high trackdensity, the film shrinks or elongates in the crosswise direction bytemperature and humidity variations, thereby causing track dislocation,or when a thin tape (base thickness of 3 to 7 μm) is caused to runrepeatedly, the end portion of the tape is damaged and deforms into aseaweed-like shape or is bent by contacting to a guide for restrictingthe transverse direction of the tape in an extreme case, thereby greatlyimpairing the characteristic properties of the tape. The Young's modulusin the transverse direction of the film is preferably 6.5 GPa or more,more preferably 7 GPa or more.

[0033] Although the Young's moduli in both longitudinal and transversedirections are desirably high, when the film is used in a magneticrecording medium of linear recording system, the Young's modulus in thelongitudinal direction is preferably higher than the Young's modulus inthe transverse direction. This is because it is more important toprevent the tape from being deformed or broken by a load as a base filmfor a high-density magnetic recording medium is thin.

[0034] The above biaxially oriented film has a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., a humidity expansion coefficient in the transversedirection (αh) of 5×10⁶/%RH to 12×10⁶/%RH and a thermal shrinkage factorat 105° C. in the transverse direction of −0.5 to +1.5%. When thetemperature expansion coefficient or the humidity expansion coefficientin the transverse direction is larger than the above range and the filmis used in a magnetic recording medium of linear recording system havinga high track density, the dimensional change in a crosswise directioncaused by temperature and humidity variations becomes large, therebycausing track dislocation and making it impossible to read data. Whenthe temperature expansion coefficient or the humidity expansioncoefficient in the transverse direction is smaller than the above range,since the Young's modulus in the transverse direction becomes high, itis difficult to retain a high Young's modulus in the longitudinaldirection. Therefore, when strong stress is applied to the magnetictape, the tape elongates and deforms disadvantageously. The thermalshrinkage factor at 105° C. in the transverse direction is preferably−0.5 to +1.0%, particularly preferably −0.5 to +0.7%. When the thermalshrinkage factor at 105° C. in the transverse direction is outside theabove range, in the step of forming a magnetic tape, the elasticity ofthe film becomes large, whereby the film may be wrinkled, coating maybecome nonuniform, or the film may not be calendered well in thecalendering step. When a magnetic tape is formed from the film, the tapeshrinks or elongates in the crosswise direction by a temperature rise inthe drive, thereby causing track dislocation and making it impossible toread data.

[0035] The above biaxially oriented film has a refractive index in thethickness direction (Nz) of preferably less than 1.490, more preferablyless than 1.487, much more preferably less than 1.485, particularlypreferably less than 1.483. When Nz is more than 1.490, surfaceorientation becomes low and it is difficult to achieve high Young'smoduli in both longitudinal and transverse directions.

[0036] The magnetic layer forming side of the above biaxially orientedfilm is preferably flat to obtain excellent electromagnetic conversioncharacteristics. The surface roughness (WRa) of at least one side of thefilm is preferably 0.5 to 10 nm, more preferably 0.8 to 7 nm,particularly preferably 1 to 5 nm. When this surface roughness WRa ishigher than 10 nm, it is difficult to maintain electromagneticconversion characteristics required for a magnetic tape. When thesurface roughness WRa is lower than 0.5 nm, the friction coefficientbecomes too large, thereby making it extremely difficult to cause thefilm to run and roll the film. The side opposite to the magnetic layerforming side, that is, non-magnetic layer side of the above biaxiallyoriented film has a surface roughness of 1 to 20 nm, more preferably 2to 15 nm, particularly preferably 2 to 12 nm to obtain excellent runningproperties. When the surface roughness of the non-magnetic layer side islower than 1 nm, the winding properties and transfer properties of thefilm during the production and processing of the film are poor, therebymaking it difficult to use it. When the surface roughness of thenon-magnetic layer side is higher than 20 nm, the flatness of themagnetic layer side may be impaired, thereby deterioratingelectromagnetic conversion characteristics.

[0037] In order to obtain surfaces which differ from each other insurface roughness, for instance, two layers which differ from each otherin the average particle diameter and amount of inert fine particles tobe added to form fine irregularities on the surface of the film may belaminated together, or a different coating layer may be formed on oneside or both sides of the film. As a matter of course, if the surfaceroughnesses of the magnetic layer side and the non-magnetic layer sidefall within the above respective ranges, the surface roughness of themagnetic layer side may be made equal to the surface roughness of thenon-magnetic layer side. In this case, a single-layer film can be easilyproduced.

[0038] A biaxially oriented film consisting of two layers is obtained byusing an unstretched laminated film consisting of two layers as theunstretched film to be stretched in the longitudinal direction in thestep (1) of the above production process of the present invention.

[0039] The inert particles to be added to the film layer on which themagnetic layer is to be formed have an average particle diameter ofpreferably 0.05 to 0.7 μm, more preferably 0.1 to 0.3 μm, particularlypreferably 0.1 to 0.2 μm. The amount of the inert particles ispreferably 0.001 to 1 wt %, more preferably 0.005 to 0.5 wt %,particularly preferably 0.01 to 0.2 wt %. When the average particlediameter of the inert particles is smaller than 0.05 μm or the amountthereof is smaller than 0.001 wt %, winding properties or transferproperties in the processing step deteriorate. When the average particlediameter is larger than 0.5 μm or the amount is larger than 1 wt %,electromagnetic conversion characteristics worsen.

[0040] Examples of the inert particles to be added to the film layer onthe magnetic layer side include (1) heat resistant polymer particles(particles of at least one of crosslinked silicone resin, crosslinkedpolystyrene, crosslinked acrylic resin, melamine-formaldehyde resin,aromatic polyamide resin, polyimide resin, polyamide-imide resin,crosslinked polyesters, etc.), and fine particles of inorganic compoundssuch as (2) metal oxides (aluminum oxide, titanium dioxide, silicondioxide (silica), magnesium oxide, zinc oxide, zirconium oxide, etc.),(3) metal carbonates (magnesium carbonate, calcium carbonate, etc.), (4)metal sulfates (calcium sulfate, barium sulfate, etc.), (5) carbon(carbon black, graphite, diamond, etc.), and (6) clay minerals (kaolin,clay, bentonite, etc.). Out of these, preferred are crosslinked siliconeresin particles, crosslinked polystyrene resin particles,melamine-formaldehyde resin particles, polyamide-imide resin particles,aluminum oxide (alumina) particles, titanium dioxide particles, silicondioxide particles, zirconium oxide particles, synthesized calciumcarbonate particles, barium sulfate particles, diamond particles andkaolin particles. More preferred are crosslinked silicone resinparticles, crosslinked polystyrene resin particles, aluminum oxide(alumina) particles, titanium dioxide particles, silicon dioxideparticles and calcium carbonate particles. The above inert particles maybe used alone or in combination of two or more.

[0041] The inert particles to be contained in the film layer on thenon-magnetic layer side have an average particle diameter of preferably0.05 to 1.0 μm, more preferably 0.1 to 0.7 μm, particularly preferably0.1 to 0.6 μm. The amount of the inert particles is preferably 0.01 to 2wt %, more preferably 0.1 to 1 wt %, particularly preferably 0.1 to 0.5wt %. When the average particle diameter is smaller than 0.05 pm or theamount is smaller than 0.01 wt %, slipperiness becomes unsatisfactoryand winding properties and handling properties in the processing stepbecome worse. When the average particle diameter is larger than 1.0 μmor the amount is larger than 2 wt %, the magnetic side becomes roughbecause the flat layer is thrust up by a lubricant contained in therough layer by calendering or the like, or the surface properties of therunning side are transferred to the magnetic side at the time of curing,thereby causing an error. The above inert particles may be used alone orin combination of two or more. As for the type of the inert particles,the same type of inert particles as those added to the magnetic layerside are preferred.

[0042] The thickness of the above biaxially oriented film is preferably2 to 10 μm, more preferably 3 to 7 μm, particularly preferably 4 to 6μm. When the thickness is larger than 10 μm, the length of the obtainedmagnetic tape wound round a cassette becomes short, thereby making itdifficult to increase the capacity of the tape. When the thickness ofthe film is smaller than 2 μm, force applied at the time of starting andstopping the magnetic tape causes the permanent elongation of the film,thereby making it difficult to obtain satisfactory durability. As forthe thickness ratio of the magnetic layer to the non-magnetic layer inthe case of a laminated film, the thickness of the non-magnetic layer ispreferably ⅔ or less, more preferably ½ or less, particularly preferably⅓ or less of the total thickness of the laminated biaxially orientedpolyester film.

[0043] Out of the above biaxially oriented films produced by the processof the present invention, the biaxially oriented film of the presentinvention is particularly excellent in physical properties as describedbelow.

[0044] The biaxially oriented polyester film has (i) a Young's modulusin the longitudinal direction of 8 to 12 GPa, (ii) a Young's modulus inthe transverse direction of 6.5 to 9 GPa, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +5×10⁻⁶/%RH to +12×10⁻⁶/%RH, and (v) a thermalshrinkage factor at 105° C. in the transverse direction of 0 to +1.5%and comprises ethylene-2,6-naphthalene dicarboxylate in an amount of atleast 95 mol % of the total of all the recurring units.

[0045] The total of Young's moduli in longitudinal and transversedirections is preferably 15 to 20 GPa.

[0046] A film having a higher Young's modulus in the longitudinaldirection than a Young's modulus in the transverse direction ispreferred.

[0047] A film having a refractive index in the thickness direction (Nz)of less than 1.490 is preferred.

[0048] Further, a film having a center plane average roughness (WRa) ofat least one side of 0.5 to 10 nm is preferred.

[0049] When the above biaxially oriented polyester film of the presentinvention is a laminated film, the laminate preferably consists of twoadjacent layers made from a polyester comprisingethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol %of the total of all the recurring units and has a center plane averageroughness (WRa) of one side of 0.5 to 10 nm and a WRa of the other sideof 1 to 20 nm.

[0050] The biaxially oriented film of the present invention can bechanged into a metal coated magnetic recording medium for-high-densityrecording which has excellent electromagnetic conversion characteristicssuch as output at a short-wavelength range, S/N and C/N, few drop outsand a low error rate by applying a coating solution prepared byuniformly dispersing iron or needle-like fine magnetic powders (metalpowders) containing iron as the main component into a binder such aspolyvinyl chloride or vinyl chloride-vinyl acetate copolymer to thesurface having a lower surface roughness (magnetic layer side) to form amagnetic layer having a thickness of preferably 1 μm or less, morepreferably 0.1 to 1 μm, and optionally further forming a back coat layeron the opposite side by a known method. A non-magnetic layer may also beformed on the surface on the magnetic layer side of the film as a layerunderlying the above metal powder-containing magnetic layer by applyinga coating solution prepared by dispersing fine titanium oxide particlesor the like in the same organic binder as that of the magnetic layer.

[0051] The thus obtained metal coated magnetic recording medium can beused as a large-capacity computer tape, particularly an LTO, DLT orSuper-DLT magnetic tape of linear recording system, for a magnetic tapewhich has excellent running properties, durability, dimensionalstability and electromagnetic conversion characteristics. In thebiaxially oriented film of the present invention, a metal thin film as amagnetic layer can be used in place of the coating film. In this case, adeposited magnetic recording medium for high-density recording which hasexcellent electromagnetic conversion characteristics such as output at ashort-wavelength range, S/N and C/N, few drop outs and a low error ratecan be obtained by forming a ferromagnetic metal thin film layer ofiron, cobalt, chromium or an alloy or oxide essentially composed thereofon the side having a lower surface roughness by vacuum vapor deposition,sputtering, ion plating or the like, forming a protective layer ofdiamond-like carbon (DLC) or the like and a fluorine-containingcarboxylic acid-based lubricant layer on the surface of theferromagnetic metal thin film layer sequentially according to purpose orapplication, and forming a back coat layer on the opposite side(non-magnetic layer) by the above method.

EXAMPLES

[0052] The following examples are provided to further illustrate thepresent invention. Various physical properties and characteristicproperties in the present invention were measured and defined asfollows.

[0053] (1) Young's Modulus

[0054] The film is cut to a width of 10 mm and a length of 150 mm, thisobtained sample is pulled by an Instron type universal tensile tester ata chuck interval of 100 mm, a pull rate of 10 mm/min and a chart rate of500 mm/min, and the Young's modulus is calculated from the tangent of arising portion of the obtained load-elongation curve.

[0055] (2) Surface Roughness (WRa)

[0056] Using the non-contact 3-D roughness meter (NT-2000) of WYKO Co.,Ltd., the surface roughness of the film is measured for 10 or more times(n) under such conditions as a measurement area of 246.6 μm×187.5 μm(0.0462 mm²) and a measurement magnification of ×25, and the centerplane average roughness (WRa) is obtained with surface analysis softwareincorporated in the roughness meter.${WRa} = {\sum\limits_{k = 1}^{m}{\sum\limits_{j = 1}^{n}{{{Z_{jk} - \overset{\_}{Z}}}/\left( {m \cdot n} \right)}}}$provided$\overset{\_}{Z} = {\sum\limits_{k = 1}^{m}{\sum\limits_{j = 1}^{n}{Z_{jk}/\left( {m \cdot n} \right)}}}$

[0057] wherein Z_(jk) is a height on a 2-D roughness chart at a j-thposition and a k-th position in a measurement direction (246.6 μm) and adirection perpendicular to the measurement direction (187.5 μm) whenthese directions are divided into m and n sections, respectively.

[0058] (3) Temperature Expansion Coefficient (αt)

[0059] The film sample is cut to a length of 15 mm and a width of 5 mmin the transverse direction of the film, and the obtained sample is setin the TMA3000 of Shinku Riko Co., Ltd. to be pre-treated at 60° C. in anitrogen atmosphere for 30 minutes and cooled to room temperature.Thereafter, the temperature is raised from 25° C. to 70° C. at a rate of2° C./min and the length of the sample is measured at each temperatureto calculate the temperature expansion coefficient (αt) of the film fromthe following equation.

αt={(L ₂ −L ₁)/(L ₀ ×ΔT)}×10⁶+0.5 (note)

[0060] wherein L₁ is the length (mm) of the sample at 40° C., L₂ is thelength (mm) of the sample at 60° C., L₀ is the initial length (mm) ofthe sample, and ΔT is 60−40=20 (° C.).

[0061] (Note): Temperature Expansion Coefficient of Quartz Glass (×10⁶)

[0062] (4) Humidity Expansion Coefficient (αh)

[0063] The film sample is cut to a length of 15 mm and a width of 5 mmin the transverse direction of the film, and the obtained sample is setin the TMA3000 of Shinku Riko Co., Ltd. and maintained at a humidity of20%RH and a humidity of 80%RH from a nitrogen atmosphere to measure thelength of the sample and calculate its humidity expansion coefficientfrom the following equation.

αh={(L ₂ −L ₁)×10⁻⁶/(L ₁ ×ΔH)}

[0064] wherein L₁ is the length (mm) of the sample at a humidity of20%RH, L₂ is the length (mm) of the sample at a humidity of 80%RH, andΔH is 60 (=80−20%RH).

[0065] (5) Thermal Shrinkage Factor

[0066] The film sample cut to a length of 300 mm and a width of 10 mm inthe transverse direction is placed in an oven heated at 105° C. under noload, heated for 30 minutes, taken out from the oven and cooled to roomtemperature to read its dimensional change. The thermal shrinkage factorof the film sample is calculated from its length (L₀) before the heattreatment and a dimensional change (ΔL) by the heat treatment based onthe following equation.

thermal shrinkage factor=(ΔL/L ₀)×100(%)

[0067] In the case of elongation, ΔL is a negative value.

[0068] (6) Refractive Index

[0069] The refractive indices in longitudinal and transverse directionsof the film are measured at 25° C. using an Abbe refractometer (of AtagoCo., Ltd.) and Na-D rays. Both the front and rear sides of the filmsample are measured and the average value of the measurement data istaken as refractive index.

[0070] (7) Dimensional Change in Crosswise Direction Under Load inLongitudinal Direction at the Time of High-Temperature and High-HumidityTreatment

[0071] The film slit to a width of 12.65 mm (½ inch) is set as shown inFIG. 1 at an ambient temperature of 23° C. and an ambient humidity of50%RH.

[0072] In FIG. 1, the numerals represent the following.

[0073]1 measurement sample

[0074]2 light emitting portion of an optical sensor (LS-3036 of KeyenceCo., Ltd.)

[0075]3 light receiving portion of an optical sensor (LS-3036 of KeyenceCo., Ltd.)

[0076]4 load

[0077]5 free roll

[0078]6 glass plate

[0079]7 measuring instrument (LS-3100 of Keyence Co., Ltd.)

[0080]8 analog/digital converter

[0081]9 personal computer

[0082]10 laser beam

[0083] Gold has been deposited on the surface of the sample slit to awidth of 12.65 mm by sputtering so that its width can be measured with adetector. In this state, a weight of 29 MPa per the sectional area ofthe film is attached to one side of the film (the other side is fixed)to measure the width (L₁) of the film with the laser outer diametermeasuring instrument of Keyence Co., Ltd. (body: model 3100, sensor:model 3060).

[0084] Thereafter, a weight of 29 MPa per the sectional area of the filmis attached to one side of the film (the other side is fixed) at atemperature of 49° C. (120° F.) and a humidity of 90%RH, kept in thisstate for 72 hours (3 days) and removed, and the film was kept at anambient temperature of 23° C. and an ambient humidity of 50% for 24hours. Then, a weight of 29 MPa per the sectional area of the film isattached to one side of the film (the other side is fixed) again tomeasure the width (L₂) of the film with the laser outer diametermeasuring instrument of Keyence Co., Ltd. (body: model 3100, sensor:model 3060).

[0085] The dimensional change in the crosswise direction (αW) before andafter the temperature and humidity treatment under load is calculatedfrom the above measured sizes before and after the temperature andhumidity treatment based on the following equation.

αW={(L ₂ −L ₁)/L ₁}×100(%)

[0086] The evaluation criteria are as follows.

[0087] (8) Track Dislocation (Error Rate)

[0088] The error rate is measured under the following conditions usingthe ML4500B QIC system of Media Logic Co.,

[0089] Ltd.

[0090] Current: 15.42 mA

[0091] Frequency: 0.25 MHz

[0092] Location: 0

[0093] Threshold: 40.0

[0094] Bad/good/max: 1:1:1

[0095] Tracks: 28

[0096] The error rate is the average value of the number of measuredtracks.

[0097] The error rate is measured under condition 1 (track dislocationcaused by temperature and humidity variations) and under condition 2(track dislocation caused by a temperature and humidity treatment) asfollows.

[0098] Condition 1 (Track Dislocation Caused by Temperature and HumidityVariations):

[0099] A tape which recorded data at 10° C. and 10%RH is reproduced at atemperature of 45° C. and a humidity of 80%RH to measure the amount oftrack dislocation caused by temperature and humidity variations. Themeasurement result is evaluated based on the amount of track dislocationof the sample of Example 1 according to the following criteria.

[0100] ⊚: error rate is zero

[0101] ◯: error rate is low and there is no practical problem

[0102] X: error rate is high and there is a practical problem

[0103] Condition 2 (Track Dislocation Caused by Temperature and HumidityTreatment)

[0104] A tape which recorded data at 23° C. and 50%RH is caused to runat 40° C. and 60%RH repeatedly for 60 hours and then kept at 23° C. and50%RH for 24 hours, and then data is reproduced at 23° C. and 50%RH tomeasure the amount of track dislocation caused by a temperature andhumidity treatment.

[0105] The measurement result is evaluated based on the amount of trackdislocation of the sample of Example 1 according to the followingcriteria.

[0106] ⊚: error rate is zero

[0107] ◯: error rate is low and there is no practical problem

[0108] X: error rate is high and there is a practical problem

[0109] (9) Electromagnetic Conversion Characteristics of Magnetic Tape

[0110] The electromagnetic conversion characteristics of a magnetic tapeare measured with the ML4500B QIC system of Media Logic Co., Ltd. Themeasurement result is evaluated based on the following criteria when theS/N of the sample of Example 1 is 0 dB.

[0111] ⊚: +1 dB or more

[0112] ◯: −1 dB or more and less than +1 dB

[0113] X: less than −1 dB

Example 1

[0114] Polyethylene-2,6-naphthalate (intrinsic viscosity: 0.6)containing 0.02 wt % of calcium carbonate particles having an averageparticle diameter of 0.6 μm and 0.2 wt % of silica particles having anaverage particle diameter of 0.1 μm was dried at 180° C. for 5 hours,melt extruded at 300° C. and solidified by quenching on a casting drummaintained at 60° C. to obtain an unstretched film.

[0115] This unstretched film was stretched to 6.2 times in alongitudinal direction between two rolls having different speeds at 150°C. The uniaxially oriented film after longitudinal stretching had arefractive index in the longitudinal direction of more than 1.77, anrefractive index in the transverse direction of 1.587 and a refractiveindex in the thickness direction of 1.534. This uniaxially oriented filmwas stretched to 4.5 times in the transverse direction at a draw rate of87.5%/sec while the temperature was raised to 120 to 155° C. in thetraveling direction of the film, further stretched to 1.1 times in thetransverse direction again at a draw rate of 2.9%/sec while thetemperature was raised to 155 to 205° C. in the traveling direction ofthe film and heat set at 190° C. in the final heat setting zone for 5seconds while it was toed out by 5% (1.05 times). The obtained biaxiallyoriented film had a thickness of 4.5 μm.

[0116] The following composition was placed in a ball mill and kneadedfor 16 hours to be dispersed, and 5 parts by weight of an isocyanatecompound (Desmodule L of Bayer AG) was added and dispersed by high-speedshearing for 1 hour to prepare a magnetic coating.

[0117] Composition of Magnetic Coating: needle-like Fe particle 100parts by weight vinyl chloride-vinyl acetate copolymer 15 parts byweight (Eslec 7A of Sekisui Chemical Co.. Ltd.) thermoplasticpolyurethane resin 5 parts by weight chromium oxide 5 parts by weightcarbon black 5 parts by weight lecithin 2 parts by weight fatty acidester 1 part by weight toluene 50 parts by weight methyl ethyl ketone 50parts by weight cyclohexanone 50 parts by weight

[0118] This magnetic coating was applied to one side of the abovebiaxially oriented PEN film to ensure that the final thickness of thecoating layer should become 0.5 μm, and the obtained film was orientedin 2,500 Gauss of a DC magnetic field, dried by heating at 100° C.,supercalendered (linear pressure of 200 kg/cm, temperature of 80° C.)and rolled. This roll was left in an oven heated at 55° C. for 3 days.

[0119] The following back coat was applied to the other side of thebiaxially oriented PEN film to ensure that the thickness of the coatshould become 1 μm, and the obtained film was dried and cut to obtain amagnetic tape.

[0120] Composition of Back Coat carbon black 100 parts by weightthermoplastic polyurethane resin 60 parts by weight isocyanate compound18 parts by weight (Colonate L of Nippon Polyurethane Kogyo Co., Ltd.)silicone oil 0.5 part by weight methyl ethyl ketone 250 parts by weighttoluene 50 parts by weight

[0121] The characteristic properties of the thus obtained film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained tape had excellent dimensional stability in the crosswisedirection (temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Example 2

[0122] The film was stretched to 6.2 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 4.2 times in the transversedirection at a draw rate of 80.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 1.20 times in the transverse direction again at a draw rateof 5.8%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film, and heat set at 190° C. in the finalheat setting zone for 5 seconds while it was toed in (0.95 time) by 5%.The obtained biaxially oriented film had a thickness of 4.5 μm.

[0123] A magnetic tape was obtained from the thus obtained film in thesame manner as in Example 1. The characteristic properties of the filmand magnetic tape are shown in Table 1. As obvious from Table 1, theobtained tape had excellent dimensional stability in the crosswisedirection (temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Example 3

[0124] Polyethylene-2,6-naphthalate (intrinsic viscosity: 0.6) for layerB which contained 0.15 wt % of crosslinked silicone resin particleshaving an average particle diameter of 0.3 μm and 0.15 wt % of sphericalsilica particles having an average particle diameter of 0.1 μm andpolyethylene-2,6-naphthalate (intrinsic viscosity: 0.6) for layer Awhich contained 0.01 wt % of spherical silica particles having anaverage particle diameter of 0.1 μm were prepared, and pellets of thesepolyethylene-2,6-naphthalates were dried at 180° C. for 5 hours,supplied to the respective hoppers of two extruders, molten at atemperature of 300° C., laminated together using a multi-manifoldcoextrusion die in such a manner that the layer A was placed on one sideof the layer B, and extruded onto a casting drum having a surface finishof about 0.3 S and a surface temperature of 60° C. to obtain anunstretched laminated film. The thickness of each layer was adjusted bythe delivery rates of the two extruders to achieve surface roughnessshown in Table 1.

[0125] This unstretched film was stretched to 6.0 times in thelongitudinal direction between two rolls having different speeds at 150°C. After longitudinal stretching, the uniaxially oriented film had arefractive index in the longitudinal direction of more than 1.77, arefractive index in the transverse direction of 1.587 and a refractiveindex in the thickness direction of 1.536. This uniaxially oriented filmwas stretched to 4.8 times in the transverse direction at a draw rate of95.0%/sec while the temperature was raised to 120 to 155° C. in thetraveling direction of the film, further stretched to 1.15 times in thetransverse direction again at a draw rate of 4.4%/sec while thetemperature was raised to 155 to 205° C. in the traveling direction ofthe film and heat set at 190° C. in the final heat setting zone for 5seconds by making the rails straight (1.00 time). The obtained biaxiallyoriented film had a thickness of 4.5 μm.

[0126] A magnetic coating was applied to the surface of the layer A(magnetic layer side) and a back coat was applied to the surface of thelayer B (non-magnetic layer side) in the same manner as in Example 1,dried and cut to obtain a magnetic tape.

[0127] The characteristic properties of the thus obtained film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained tape had excellent dimensional stability in the crosswisedirection (temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Comparative Example 1

[0128] The film was stretched to 6.2 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 2.6 times in the transversedirection at a draw rate of 40.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 2.00 times in the transverse direction again at a draw rateof 29.2%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film and heat set at 190° C. in the finalheat setting zone for 5 seconds by making the rails straight (1.00time). Since the film was often broken by the second transversestretching, a roll sample could not be obtained.

Comparative Example 2

[0129] The film was stretched to 6.0 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.536. Thisuniaxially oriented film was stretched to 5.5 times in the transversedirection at a draw rate of 112.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). Since the film was often brokenby the first transverse stretching, a roll sample could not be obtained.

Comparative Example 3

[0130] The film was stretched to 6.2 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 4.3 times in the transversedirection at a draw rate of 82.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 1.10 times in the transverse direction again at a draw rateof 2.9%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film and heat set at 190° C. in the finalheat setting zone for 5 seconds while it was toed out by 10% (1.10times). The obtained biaxially oriented film had a thickness of 4.5 μm.

[0131] A magnetic tape was obtained from the thus obtained film in thesame manner as in Example 1. The characteristic properties of the filmand magnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a large thermal shrinkage factor in the transversedirection and poor dimensional stability in the crosswise direction ofthe tape (high-temperature and high-humidity treatment under load inlongitudinal direction).

Comparative Example 4

[0132] The film was stretched to 5.7 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.539. Thisuniaxially oriented film was stretched to 3.9 times in the transversedirection at a draw rate of 72.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). The obtained biaxially orientedfilm had a thickness of 4.5 μm.

[0133] A magnetic tape was obtained from the thus obtained film in thesame manner as in Example 1. The characteristic properties of the filmand magnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a low Young's modulus in the transverse direction andpoor dimensional stability in the crosswise direction of the tape(temperature and humidity variations).

Comparative Example 5

[0134] The film was stretched to 4.0 times in the longitudinal directionat 150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.558. Thisuniaxially oriented film was stretched to 5.4 times in the transversedirection at a draw rate of 110.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). The obtained biaxially orientedfilm had a thickness of 4.5 μm.

[0135] A magnetic tape was obtained from the thus obtained film in thesame manner as in Example 1. The characteristic properties of the filmand magnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a low Young's modulus in the transverse direction andpoor dimensional stability in the crosswise direction of the tape(high-temperature and high-humidity treatment under load in longitudinaldirection). TABLE 1 Item unit Ex. 1 Ex. 2 Ex. 3 C. Ex. 1 C. Ex. 2 C. Ex.3 C. Ex. 4 C. Ex. 5 layer structure single layer single layer doublelayers single layer single layer single layer single layer single layerfilm forming conditions draw ratio in longitudinal times 6.2 6.2 6.0 6.26.0 6.2 5.7 4.0 direction draw ratio in transverse direction first drawratio (SD1) times 4.5 4.2 4.8 2.6 5.5 4.3 3.9 5.4 second draw ratio(SD2) times 1.10 1.20 1.15 2.00 1.00 1.10 1.00 1.00 ratio in final heatsetting times 1.05 0.95 1.00 1.00 1.00 1.10 1.00 1.00 zone (SD3) totalratio times 5.20 4.79 5.52 5.20 5.50 5.20 3.9 5.40 (SD1 × SD2 × SD3)draw rate first draw rate (S1) %/sec 87.5 80.0 95.0 40.0 112.5 82.5 72.5110.0 second draw rate (S2) %/sec 2.9 5.8 4.4 29.2 0.0 2.9 0.0 0.0 S1/S2— 0.03 0.07 0.05 0.73 0.00 0.04 0.00 0.00 film formation state firsttransverse stretching film is often zone broken second transversestretching film is often zone broken Item unit Ex. 1 Ex. 2 Ex. 3 C. Ex.1 C. Ex. 2 C. Ex. 3 C. Ex. 4 C. Ex. 5 Physical properties Film thicknessμm 4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Young's modulus Longitudinaldirection GPa 9.0 9.7 8.5 9.0 8.5 9.0 9.5 6.0 Transverse direction GPa7.0 6.5 7.5 7.0 7.5 7.0 5.5 9.0 Refractive index in thickness direction(biaxially 1.484 1.487 1.484 1.484 1.484 1.484 1.487 1.487 orientedfilm) Temperature expansion coefficient (αt) ×10⁻⁶/° C. 3 8 0 3 0 3 17−5 Transverse direction Humidity expansion coefficient (αh) ×10⁻⁶/% RH12 13 10 12 10 12 16 8 Transverse direction 105° C. thermal shrinkagefactor % 1.3 0.3 0.8 2.3 0.5 1.8 0.3 0.3 transverse directiondimensional change in crosswise direction under load % 0.20 0.17 0.220.20 0.22 0.20 0.18 0.35 in longitudinal direction caused byhigh-temperature and high-humidity treatment surface roughness magneticlayer side (WRa) nm 6 6 2 6 6 6 6 6 non-magnetic layer side nm 6 6 8 6 66 6 6 track dislocation condition 1 (track dislocation caused bytemperature ⊚ ◯ ⊚ — — ⊚ X ⊚ and humidity variations) condition 2 (trackdislocation caused by temperature ◯ ⊚ ◯ — — X ⊚ X and humidityvariations) electromagnetic conversion characteristics ◯ ◯ ⊚ — — ◯ ◯ ◯

1. A process for producing a biaxially oriented polyester film,comprising the steps of: (1) stretching an unstretched film of apolyester which comprises ethylene-2,6-naphthalene dicarboxylate in anamount of at least 95 mol % of the total of all the recurring units to4.5 to 7.0 times in a machine direction at a temperature of 100 to 190°C. to form a uniaxially oriented film; and (2) stretching thisuniaxially oriented film to 3.0 to 6.0 times in a transverse directionat a temperature of 110 to 170° C. while raising the temperature in thetraveling direction of the film and then stretching the film to 1.05 to1.5 times at a lower draw rate than the first draw rate at a temperaturefrom the final temperature of the first transverse stretching to 240° C.while raising the temperature in the traveling direction of the film toform a biaxially oriented film having (i) a Young's modulus in thelongitudinal direction of 8 GPa or more, (ii) a Young's modulus in thetransverse direction of 6 GPa or more, (iii) a temperature expansioncoefficient (αt) in the transverse direction of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient (αh) in thetransverse direction of +5×10⁻⁶/%RH to +12×10⁻⁶/%RH, and (v) a thermalshrinkage factor at 105° C. in the transverse direction of −0.5 to+1.5%.
 2. The process of claim 1, wherein longitudinal stretching inmachine direction of the step (1) is carried out at a temperature of 120to 170° C.
 3. The process of claim 1, wherein the draw ratio ofstretching in machine direction of the step (1) is 5.0 to 6.5 times. 4.The process of claim 1, wherein the uniaxially oriented film formed inthe step (1) has a refractive index in the longitudinal direction of1.77 or more, a refractive index in the transverse direction of 1.55 to1.62 and a refractive index in the thickness direction of 1.50 to 1.56.5. The process of claim 1, wherein the first transverse stretching inthe step (2) is carried out at a temperature of 130 to 160° C.
 6. Theprocess of claim 1, wherein the draw ratio of the first transversestretching in the step (2) is 4.0 to 5.0 times.
 7. The process of claim1, wherein the second transverse stretching in the step (2) is carriedout at the final temperature of the first transverse stretching to 220°C.
 8. The process of claim 1, wherein the draw ratio of the secondtransverse stretching in the step (2) is 1.05 to 1.2 times.
 9. Theprocess of claim 1, wherein the gradient of a temperature rise in thetraveling direction of the film in the first transverse stretching ofthe step (2) is within the range of 15 to 55° C.
 10. The process ofclaim 1, wherein the gradient of a temperature rise in the travelingdirection of the film in the second transverse stretching of the step(2) is within the range of 20 to 90° C.
 11. The process of claim 1 whichfurther comprises the step of shrinking or stretching the film to 0.9 to1.05 times in the transverse direction at the same temperature as thefinal temperature of the second transverse stretching or at atemperature of 170 to 230° C. while reducing the temperature from thefinal temperature in the traveling direction of the film after thesecond transverse stretching.
 12. A biaxially oriented polyester filmhaving (i) a Young's modulus in a longitudinal direction of 8 to 12 GPa,(ii) a Young's modulus in a transverse direction of 6.5 to 9 GPa, (iii)a temperature expansion coefficient in the transverse direction (αt) of−5×10⁻⁶/° C. to +12×10⁻⁶/° C., (iv) a humidity expansion coefficient inthe transverse direction (αh) of +6×10⁻⁶/%RH to +12×10⁻⁶/%RH and (v) athermal shrinkage factor at 105° C. in the transverse direction of 0 to+1.5% and comprising (vi) ethylene-2,6-naphthalene dicarboxylate in anamount of at least 95 mol % of the total of all the recurring units. 13.The film of claim 12 which has a total of Young's moduli in longitudinaland transverse directions of 15 to 20 GPa.
 14. The film of claim 12which has a higher Young's modulus in the longitudinal direction than aYoung's modulus in the transverse direction.
 15. The film of claim 12which has a refractive index in the thickness direction (Nz) of lessthan 1.490.
 16. The film of claim 12 which has a center plane averageroughness (WRa) of at least one side of 0.5 to 10 nm.
 17. The film ofclaim 12 which is a laminate comprising two layers of a polyestercomprising ethylene-2,6-naphthalene dicarboxylate in an amount of atleast 95 mol % of the total of all the recurring units and has a centerplane average roughness (WRa) of one side of 0.5 to 10 nm and a WRa ofthe other side of 1 to 20 nm.
 18. Use of the biaxially orientedpolyester film of claim 12 as a base film for a magnetic recordingmedium.
 19. Use of claim 18, wherein the magnetic recording medium is adigital recording medium of linear recording system.
 20. Use of claim18, wherein the magnetic recording medium is a coated magnetic recordingmedium.
 21. Use of claim 18, wherein the magnetic recording medium is aferromagnetic metal thin film magnetic recording medium.
 22. A magneticrecording medium comprising the biaxially oriented polyester film ofclaim 12 and a magnetic layer formed on the surface of the film.